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(11) | EP 0 428 924 A3 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Frequency detector for discriminating multi-longitudinal mode laser operation |
| (57) A photodetector produces an output signal in response to irradiation by an optical
beam from a pulled laser. A delay unit delays the output signal for a period corresponding
to one-half of one cycle of a beat frequency generated by interaction of two frequency
components in the optical beam corresponding to two predetermined longitudinal modes
of operation of the laser. A differential mixer subtractively combines the output
of the photodetector with the output of the delay unit, to produce an output signal
having the main profile of the laser pulses suppressed, and the beat frequency component,
if present, amplified. A resonant circuit and amplifier may be provided to further
amplify the beat frequency component. The output of the amplifier is integrated to
produce a DC signal having a magnitude which corresponds to the amplitude of the beat
frequency component. The DC signal is compared with a threshold value to discriminate
between single and multi-mode operation of the laser. An optical shutter may be closed
in response to sensed multi-longitudinal mode operation to block the laser beam and
protect equipment which would otherwise be damaged thereby. |